Liquid crystal displays (LCD's) are commonly used in television receivers, portable computer displays, and other electronic devices. An LCD requires a source of backlight for operation because the LCD operates effectively as a light valve, allowing transmission of light in one state and blocking transmission of light in a second state. The typical LCD panel structure includes a liquid crystal polymer encapsulated between at least two planar glass plates in parallel with each other. A polarization layer is bonded to the outer surface of each glass plate such that the glass plates are sandwiched between two polarization layers.
The inner surface, or the surface facing the liquid crystal polymer, of each glass plate includes mutually perpendicularly oriented, conductive, transparent linear arrays of electrodes. The volume of the liquid crystal polymer between any two orthogonal arrays of electrodes forms a cube whose face area constitutes a pixel. The electrode arrays are connected on the periphery of the glass plates via input/output (I/O) strips for coupling to electronic circuitry for applying a voltage to the two sets of orthogonal conductive transparent electrode arrays. To one set of linear, parallel arrays of electrodes is provided video image information (typically to the vertically aligned electrodes), while to the other linear, parallel array of electrodes (typically the array oriented horizontally) are provided "ON" signals in a sequential manner such that each horizontal linear array of electrodes is sequentially turned on rendering the portion of the liquid crystal polymer adjacent the turned-on electrode array transparent for presenting a portion of the video image. The vertically aligned electrodes are known as "signal" electrodes, while the horizontally aligned electrodes are commonly referred to as "scanning" electrodes.
Referring to FIGS. 1a and 1b, the basic operating principal of an LCD is shown in simplified schematic diagram form. As shown in FIG. 1a, the liquid crystal polymer 10 is disposed intermediate first and second transparent electrodes 12 and 14. The aforementioned polarization layers on each side of the liquid crystal polymer 10 are omitted from the figures for simplicity. The liquid crystal polymer 10 is illustrated in simplified form as comprised of a plurality of molecules 16. A voltage source 20 couples the first and second transparent electrodes 12, 14 via a switch 18 which is shown open in FIG. 1a. Without a voltage applied across the liquid crystal polymer 10, the liquid crystal molecules 16 are randomly oriented as shown in FIG. 1a and incident light (shown in the figure as solid arrows) is randomly scattered by the liquid crystal polymer 10 as shown by the arrows in dotted-line form. With switch 18 closed and a voltage applied across the liquid crystal polymer 10, the liquid crystal molecules 18 become aligned and render the liquid crystal polymer transparent. Thus, light incident upon the liquid crystal polymer 10 is transmitted therethrough as shown in FIG. 1b.
In a conventional projection type LCD display, such as shown in simplified schematic diagram form in FIG. 2 as element 26, a light source 28 is placed behind the LCD panel 30 to illuminate the panel and project a video image. Light source 28 may be a fluorescent discharge tube or a metal halide or Xenon arc lamp. The projected video image from LCD panel 30 is focused by means of a lens 32 and directed through an aperture 34a in an aperture stop 34. A projection lens unit 36 then projects the video image onto the aft surface of a projection screen 38.
At any point during operation, the LCD panel 30 has only one horizontal line "ON," with only this line allowing for transmission of light through the panel. Facing electrodes adjacent all other horizontal lines in the LCD panel 30 are OFF. Therefore, most of the light from the light source 28 is blocked by the non-transmitting portions of the LCD panel and converted to heat. This arrangement is characterized by low light utilization efficiency, the generation of a substantial amount of heat which must be dissipated, and limited brightness and contrast because of the substantial portion of backlighting which is scattered by the non-transparent portion of the LCD panel. With 525 horizontal scan lines in a conventional projection LCD system, it can be seen that with only 1/525th of the input light utilized for producing a useful image, much energy is wasted in a conventional projection LCD system. This waste will increase in future systems employing high definition television (HDTV) displays which employ over 1000 horizontal scan lines and which will reduce light utilization efficiency even further. In order to increase image brightness and contrast, more powerful light sources are being developed. These more powerful light sources providing more lumens per watt will produce even more heat and make component cooling and heat dissipation even more important design considerations.
This invention addresses the aforementioned limitations of the prior art by providing a backlighting source for a projection LCD system which brightly illuminates only that portion of the LCD panel rendered transparent and containing video information and which maintains the backlighting beam on the transparent portion as it is displaced on the display panel.